Add and use SDL_FALLTHROUGH for fallthroughs

Case fallthrough warnings can be suppressed using the __fallthrough__
compiler attribute. Unfortunately, not all compilers have this
attribute, or even have __has_attribute to check if they have the
__fallthrough__ attribute. [[fallthrough]] is also available in C++17
and the next C2x, but not everyone uses C++17 or C2x.

So define the SDL_FALLTHROUGH macro to deal with those problems - if we
are using C++17 or C2x, it expands to [[fallthrough]]; else if the
compiler has __has_attribute and has the __fallthrough__ attribute, then
it expands to __attribute__((__fallthrough__)); else it expands to an
empty statement, with a /* fallthrough */ comment (it's a do {} while
(0) statement, because users of this macro need to use a semicolon,
because [[fallthrough]] and __attribute__((__fallthrough__)) require a
semicolon).

Clang before Clang 10 and GCC before GCC 7 have problems with using
__attribute__ as a sole statement and warn about a "declaration not
declaring anything", so fall back to using the /* fallthrough */ comment
if we are using those older compiler versions.

Applications using SDL are also free to use this macro (because it is
defined in begin_code.h).

All existing /* fallthrough */ comments have been replaced with this
macro. Some of them were unnecessary because they were the last case in
a switch; using SDL_FALLTHROUGH in those cases would result in a compile
error on compilers that support __fallthrough__, for having a
__attribute__((__fallthrough__)) statement that didn't immediately
precede a case label.

src/audio/SDL_wave.c

@@ -1715,7 +1715,7 @@ WaveCheckFormat(WaveFile *file, size_t datalength)
         if (file->facthint == FactStrict && file->fact.status <= 0) {
             return SDL_SetError("Missing fact chunk in WAVE file");
         }
-        /* fallthrough */
+        SDL_FALLTHROUGH;
     case PCM_CODE:
         /* All supported formats require a non-zero bit depth. */
         if (file->chunk.size < 16) {
@@ -1854,7 +1854,7 @@ WaveLoad(SDL_RWops *src, WaveFile *file, SDL_AudioSpec *spec, Uint8 **audio_buf,
             RIFFend = RIFFchunk.position + SDL_MAX_UINT32;
             break;
         }
-        /* fallthrough */
+        SDL_FALLTHROUGH;
     case RiffSizeForce:
         RIFFend = RIFFchunk.position + RIFFchunk.length;
         RIFFlengthknown = SDL_TRUE;